Use of silica fume and natural volcanic ash as a replacement to Portland cement: Micro and pore structural investigation using NMR, XRD, FTIR and X-ray microtomography
Introduction
Volcanic ash (VA) has been used in the past as an admixture for concrete applications [1], [2], [3], [4]. The Romans were the first to use natural aluminosilicates to prepare highly durable cements [5]. Volcanic materials are found abundantly in areas around the world, and new and improved ways to utilize these materials in construction is becoming widespread. The motivation for the use of volcanic ash materials as replacement of Portland cement is due to the significant carbon footprint of concrete materials [6], [7], [8], [9], [10], the regional availability of VA, and its capability to be used as an additive for high-performance materials [3], [5], [11].
There are two types of VA, one which erupts from molten rock and primarily consists of basaltic compositions, and the other type which originates from the more explosive pyroclastic flow eruptions that develop to form secondary pozzolanic clays and zeolitic phases [3], [11], [12]. Recent studies have utilized the basaltic ash type as a supplementary cementitious material in concrete and found that it complies with ASTM C 618 and is categorized as a class N natural pozzolana [4]. Further work is needed to better understand the factors controlling the incorporation of VA within a concrete mixture as well as the reaction mechanisms controlling hydration.
Not all ash materials are suitable for usage within blended cements [13], [14]. The ability of an ash material to replace Portland cement is partly a function of its pozzolanic activity, which depends on the amount of reactive silica and the amorphous content of the ash [3]. A recent study by Contrafatto [14] suggests that the pyroclasts produced by the explosive activity of Mt. Etna, Italy did not have sufficient reactive SiO2 hence unable to produce sufficient pozzolanic reactivity to achieve the required strength and thus could not be used as an SCM for producing cement pastes. Several methods of increasing the overall pozzolanic reactivity have been investigated. This includes reducing the mean size of ash particles to provide a higher specific surface area, or by reducing the quantity of clay minerals while increasing the zeolitic mineral composition in the ash [15]. In one study, vibratory milling was used to increase the amorphous composition which resulted in higher pozzolanic activity [16]. In addition, the pozzolanic activity is enhanced by the amount of cations present in the aluminosilicate precursor and the ease with which these cations are exchanged [17]. Calcination, acid [18], [19] and thermal treatments [20] have also been used to assist with the preparation of blended cements.
These blended cement pastes prepared with Supplementary Cementitious Materials (SCM) form a complex composite of hydration product, and to decipher this binder a multi-scale analysis using advanced experimental techniques is required. The microstructural growth of these hydration products is at intermittent scales ranging from angstrom to microns, hence; a combination of experimental techniques is required for understanding evolution and growth of these hydration products [21]. Pure OPC and Tricalcium Silicate (C3S) mixtures have been well studied using advanced micro and pore structural techniques such as Magic Angle Spin (MAS) Nuclear Magnetic Resonance (NMR) [22], [23], [24] and X-ray Microtomography (X-ray μCT) [25], [26], [27], [28], [29], [30]. However, limited data is available using these characterization techniques when SCM’s are used with natural pozzolans (volcanic ash) for preparing sustainable and durable cement pastes [31], [32], [33], [34], [35], [36], [37], [38], [39]. These studies mostly focus only on usage of volcanic ash with Portland cement, however, considering the current needs incorporation of silica fume with volcanic ash is required, since silica fume is currently being commercially used for developing high strength concretes [40].
For this study, we will evaluate the mineralogical nature of VA as a partial substitute to Portland cement, and assess the resulting hydration products of various cement paste mixtures incorporating the silica fume additive. VA was obtained from a pozzolan factory located in Jeddah, Saudi Arabia. Our investigation utilizes experimental techniques of NMR, X-ray microtomography, X-ray Diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR) and nitrogen adsorption to study the feasibility of VA as a partial replacement for Portland cement to obtain denser and less porous cementitious microstructures. This study examines the effect of silica fume, along with VA, by examining the type and nature of hydration products using multiple micro-characterization techniques. A new insights into micro- and pore-structure formation is observed as a basis for developing engineered cement pastes when Portland cement is partially replaced with VA and silica fume. Silica fume is a common additive used with OPC for densifying the matrix; however, the current study investigates the interaction of silica fume with VA in terms of hydration products that influences the pore structure of the resulting cementitious binder.
A microstructural insight from angstrom level using XRD and the hydration products formation was examined via 29Si and 27Al NMR along with bonding mechanism was studied via FTIR analysis. The pore structure and porosity was examined by X-ray microtomography. Furthermore, adsorption-desorption isotherms and pore size distribution was examined via nitrogen sorptiometry. These new findings provide information using multi-scale techniques from angstrom to micro-meter level for analyzing the pore and microstructure of cement pastes when volcanic ash and silica fume are used as replacement to Portland cement.
Section snippets
Sample preparation
Finely ground VA was procured from Harrat Rahat, Jabal Kadaha quarry, Medina Province, Saudi Arabia. In addition to VA, Type I Ordinary Portland Cement (OPC) and silica fume (SF) were used in this study. The combinations of OPC, SF and VA considered are shown in Table 1. A constant water to cement ratio of 0.45 was maintained for all samples which were lime cured at ambient temperature for 28 days. The control mix consisted of only OPC with no additions of VA or SF. For convention, we label each
XRD analysis
XRD diffractograms for the raw VA and OPC materials, as well as blends of OPC, VA and SF are shown in Fig. 7, Fig. 8. Cement pastes prepared with the combination of OPC, VA and SF were cured for 28 days. The original composition of the VA consisted of higher traces of MgO (6.16%), Al2O3 (13.00%) and SiO2 (38.89%) as shown in Table 2. XRD analysis of VA revealed traces of Anorthite sodian , Frosterite and Wairakite ]. These phases were partially
Summary of findings
A laboratory study was conducted to investigate the pore and microstructure resulting from high volume replacement of Portland cement with ground VA from Saudi Arabia. We used multiple experimental techniques and observed the following:
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XRD results indicate that heulandite-Ca and gismondine were associated with the formation of C-A-S-H gels, while α-C-S-H and oyelite was associated with the formation of calcium silicate hydrate (C-S-H). Addition of VA led to formation of magnesium related phases
Conclusion
This work investigates the effectiveness of the use of basaltic volcanic ash as a partial replacement for Portland cement. Multiple mix combinations of volcanic ash, silica fume and Portland cement were examined using various pore and microstructure characterization techniques. The results indicate that the addition of silica fume helped to decrease the porosity and allowed for up to 40% substitution for OPC. The high alumina content in the ash reacted with additional silica from the silica fume
Acknowledgements
This project was sponsored by the Kuwait Foundation for the Advancement of Sciences. The project was conducted as part of the Kuwait-MIT signature project on sustainability of Kuwait’s built environment under the direction of Professor Oral Büyüköztürk. Use of the Advanced Photon Source (APS), an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. We
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